In an electrophotographic modular printing machine of known type, such as for example the NexPress 2100 printer manufactured by NexPress Solutions, Inc., based in Rochester, N.Y., color toner images are made sequentially in a plurality of color imaging modules arranged in tandem, and the toner images are successively electrostatically transferred to a receiver sheet adhered to a transport web moved through the modules. Commercial machines of this type typically employ intermediate transfer members in the respective modules for the transfer to the receiver member of individual color separation toner images. However, the invention as described herein also contemplates the use of tandem electrostatographic printers that do not employ intermediate transfer members but rather transfer each color separation toner image directly to the receiver member.
Electrostatographic printers having a four-color capability are known to also provide a fifth toner depositing station for depositing for example, clear toner. The provision of a clear toner overcoat to a color print is desirable for providing protection of the print from fingerprints and reducing certain visual artifacts. However, a clear toner overcoat may add cost and may reduce color gamut of the print so it is desirable to provide for operator/user selection to determine whether or not a clear toner overcoat will be applied to the entire print. In U.S. Pat. No. 5,234,783, (Ng) it is noted that in lieu of providing a uniform layer of clear toner that a layer that varies inversely according to heights of the toner stack may be used instead as a compromise approach to even toner stack heights. As is known, the respective color toners are deposited one upon the other at respective locations on the receiver and the height of a respective color toner stack comprises the sum of the toner contributions of each respective color.
The invention recognizes that a four-color process provides a color gamut that is relatively limiting. The invention further recognizes that in using a tandem printer apparatus with five printing stations or modules one can unexpectedly still achieve an improved color gamut with application of clear toner in accordance with the teachings set forth herein.